The field of the present invention relates to hydraulic lifters or tappets for internal combustion engines, particularly such lifters or tappets which are substantially horizontally oriented.
Prior known engines that contain hydraulic lifter assemblies for actuating cylinder intake and exhaust valves are well known in the art. The camshaft contains one or more lobes which slidably engage the foot of the lifter, and force open valves which are spring-biased into their closed positions. Gases are conducted into, or exhausted from, the combustion chamber of the engine past the valves, which are fitted into ports provided in a cylinder head or the cylinder block. The combustion chamber normally includes a cylinder, over which the cylinder head is disposed, and in which a piston reciprocates; the piston is operably coupled to a rotatable crankshaft, which has an axis of rotation normally parallel with that of the camshaft. In the well known manner, combustion within the chamber forces the piston away from the head, driving the crankshaft. In a single cylinder engine, angular inertia of the crankshaft causes the reciprocating piston to approach the head. In multiple cylinder engines, the reciprocating piston is also urged toward the head under power of the crankshaft as pistons in other cylinders are similarly urged away from their respective heads during combustion therein.
The camshaft is driven by the crankshaft, and may be operably coupled by means of a gear drive, a belt drive or a chain drive, all of which are well known in the art and provide the proper camshaft/crankshaft drive ratio. In a four stroke engine, the crankshaft rotates twice for each rotation of the camshaft, and each piston successively undergoes compression, power, exhaust and intake strokes in each cycle. Ignition of a fuel/air mixture within the combustion chamber, which causes combustion therein, occurs at or near the beginning of the power stroke. During the compression and power strokes, the exhaust and intake valves are normally closed. During the exhaust stroke or intake stroke, the respective exhaust valve or intake valve is open, and gas respectively exits or enters the cylinder past the valve. The timing and duration of the valve openings and closings, as well as the distance by which the valve is opened, is controlled by the profile of the cam lobes.
In two stroke engines, each piston successively undergoes compression/exhaust and power/intake strokes in each cycle. During a portion of the compression stroke, the exhaust valve is open; during a portion of the power stroke, the suction valve is open. Thus, for each rotation of the camshaft, in a two stroke engine the crankshaft rotates once.
As noted above, the foot of each lifter rides on a cam lobe. As the lifter foot follows the profile of the cam lobe, the spring-biased valve is forced off of its seat, which surrounds the respective port, to open the valve, and is allowed to return to its seat to close the valve. The valves and the camshaft lobes are thus operably engaged through the lifters, as well as through any intervening valve rods or rocker arm assemblies which are also included in the valve train to manipulate the direction of motion and/or proportionally change the amount of lift imparted to the valve, as known in the art.
Lifters or tappets are generally of two types: Solid and hydraulic. Solid lifters are comparatively cheaper, include fewer components, and offer a somewhat greater degree of control over valve travel because they do not compress. Solid lifters, however, require periodic adjustment to maintain proper valve train operating tolerances. When out of tolerance, solid lifters are prone to cause undesirable noise during engine operation as the lifter foot and cam lobes, or other parts of the valve train, slightly separate and subsequently strike each other.
Hydraulic lifters are comparatively more expensive than solid lifters and include more component parts. Nevertheless, they are virtually maintenance-free and normally very quiet. Further, in most engines, hydraulic lifters offer a satisfactory degree of control over valve travel, despite their being compressible.
The hydraulic lifter assembly may comprise an elongate, usually cylindrical body, closed on one end by the portion forming the foot of the lifter. The lifter body is normally slidably disposed in a bore provided in the cylinder block or valve head which extends perpendicularly relative to the axis of rotation of the camshaft. The foot of each lifter body rides on the profile of a different cam lobe as the lifter body reciprocates within its bore.
A hollow plunger, also usually cylindrical, is slidably disposed within the lifter body, and is spring-biased away from the foot. The end of the plunger opposite the foot of the lifter body engages its valve, perhaps, as mentioned above, through valve rods and/or rocker arms. The plunger contains a low pressure reservoir into which engine oil is received. The lifter body has a high pressure oil, expansible reservoir located between the foot and the plunger. The high pressure reservoir is in one-way fluid communication with, and receives oil from, the low pressure reservoir through a check valve.
During operation, as the highest part or peak of the rotating cam lobe moves out from under the foot of the lifter body, and the lifter body consequently advances radially toward the axis of rotation of the camshaft, the spring within the lifter assembly forces the foot away from the plunger, and oil from the low pressure reservoir is drawn through the check valve into the high pressure reservoir, thereby fully charging the high pressure reservoir with oil as the lifter foot encounters the base or circular portion of the cam lobe. As the lifter foot encounters the ramp portion of the cam lobe which extends from the base to the peak, the lifter body is forced radially away from the axis of rotation of the camshaft. The lifter assembly spring and the oil in the high pressure reservoir is compressed, and the plunger forces the valve open. The compressed oil in the high pressure reservoir is forced therefrom through clearances between the valve body and the plunger, and subsequently from between the valve body and the bore in which it reciprocates. Thus, a hydraulic lifter forms a dashpot.
As the reciprocating lifter body again advances towards the axis of rotation of the camshaft, oil is again drawn from the low pressure reservoir to the high pressure reservoir as the lifter assembly spring forces the lifter body and plunger axially apart, and the cycle continues.
To ensure quiet and reliable operation of the lifter assembly, it is important that an adequate supply of oil be provided to both the low and high pressure reservoirs at all times. A problem often encountered is that, during engine shutdown periods, oil will leak or drain from the reservoirs of the lifter assemblies. This leakdown phenomena is particularly common in engines which have horizontally-oriented lifter assemblies. Vertically-oriented lifter assemblies do not experience this problem to the same degree as horizontally-oriented lifter assemblies do, because the lifter bodies of most vertically-oriented lifter assemblies are closed by foot-forming lower portions and therefore have a tendency to retain oil therein.
Upon subsequent startup of engines having previous horizontally-oriented lifter assemblies, at least the high pressure reservoirs, and perhaps also the low pressure reservoirs, of the lifter assemblies may be depleted of oil and largely contain air. Consequently, these lifter assemblies compress too readily and too far, resulting in undesirable noise or improper valve timing, at least temporarily, as well as possible damage to components of the valve train (including the lifter assemblies themselves). Thus, a hydraulic lifter assembly which precludes oil from leaking therefrom, and air from entering thereinto, during engine shutdown periods is highly desirable.
The present invention addresses the above-mentioned leakdown problem by providing a hydraulic lifter assembly which allows no oil to leak out of its reservoirs during engine shutdown periods.
The present invention provides a lifter assembly for an internal combustion engine, including an elongate sleeve having an upper radial wall portion provided with oil feed and oil bleed holes therethrough, through which oil respectively enters and exits the lifter assembly. An elongate, hollow lifter body is reciprocatingly disposed within the sleeve, the lifter body being closed at one end thereof. A plunger is reciprocatingly disposed within the lifter body and has an internal cavity, a low pressure oil reservoir at least partially defined by the plunger internal cavity, the low pressure oil reservoir in at least periodic fluid communication with the oil feed hole, whereby oil from the oil feed hole is received into the low pressure oil reservoir. A high pressure oil reservoir is at least partially defined by the plunger and the lifter body closed end, and is in one-way fluid communication with the low pressure oil reservoir, whereby oil is received into the high pressure reservoir from the low pressure oil reservoir. A cap is reciprocatingly disposed within the sleeve and engaged with the plunger. First and second seals are located between an outer circumferential surface of the lifter body and an outer circumferential surface of the cap, respectively, and an inner circumferential surface of the sleeve, the seals respectively located between the oil feed hole and one end of the sleeve, and the oil bleed hole and the other end of the sleeve, whereby oil is precluded from exiting the lifter assembly through the ends of the sleeve.
The present invention also provides an arrangement of first and second lifter assemblies, wherein each of the first and second lifter assemblies includes a sleeve in which a lifter body reciprocates, each sleeve provided with an oil feed hole through which oil is provided to the lifter body therein. The oil feed holes of the first and second lifter assemblies are in parallel fluid communication with a source of oil which includes a first oil conduit. A circumferential groove is located about the first lifter assembly sleeve, the oil feed hole of the first lifter assembly sleeve opening into the circumferential groove, and a second conduit is provided through which the circumferential groove and the second lifter assembly oil feed hole are placed in fluid communication.